Notes

The spectral composition of illumination represents the set of wavelengths present in light source. Each surface has a particular reflectance spectrum, not all surfaces reflect light in the same way. The product of illumination and reflectance gives us the color signal, i.e., the color that arrives at the eye, which is a combination of which wavelengths are generated by the source of illumination and which wavelengths are reflected by the surface. The color signal that reaches the eye is then filtered by the three populations of cones receptors present in the retina. Hence, the signal representing colors that is propagated as information in the brain can be summarized by three numbers, i.e., how much activity the signal generated in the Long (Red), Medium (Green) /Short (Blue) wavelength sensitive cones. Keep in mind, that the wavelength composition of the color signal is not sufficient to assess with certainty the color appearance in the eye. Indeed, adaptation, context, background and various factors affect the color perception.

Photoreceptors Sensitivity Profiles The picture below shows the same concept as before. The curves show the relative absorbance of wavelengths depending on the cones/rods type. It is important that also the rods are wavelength selective, even though they are mostly used in low-lighting settings. Since we only have three cones type, our vision is defined trichromatic. Side note: computer screens feature only three types of pixels matching the three wavelengths preferences of cones photoreceptors.

About 10% of human population suffers from color blindness. People with color blindness have a restricted spectrum of colors. Color blindness occurs when one or more types of cone cells in the retina are absent, non-functioning, or have an altered response to light. In a person with normal color vision, each cone type is most sensitive to a specific range of wavelengths. When light enters the eye and stimulates these cones, the brain integrates the signals from each cone type to perceive color. In individuals with color blindness, one or more of the cone types are either absent or have an altered response to light. This can result in an inability to distinguish certain colors or a reduced ability to see certain color combinations. Color blindness can be inherited or acquired. Inherited color blindness is usually due to a genetic mutation that affects the development or function of the cone cells. Acquired color blindness can occur due to certain diseases or conditions that damage the retina or optic nerve, or due to exposure to certain chemicals or medications that can affect the function of the cone cells.